Since it was shown by Kohler and Milstein (Nature Vol. 256, 495-497, 1975) that it was possible to fuse mouse myeloma cells with spleen cells from immunized mice and thereby produce a continuous cell line which produces a homogeneous (monoclonal) antibody, extensive attention has been focused on the production of these hybrid cell lines (hybridomas) and the monoclonal antibodies (Mabs) produced.
The development of hybridoma technology has led to a drammatically improved understanding of the antigenic molecules on the surface of human leucocytes. Much of this information has recently been systematised in the International Workshop on Human Leucocyte Differentiation Antigens, and the majority of many thousands of Mabs reported can now be recognised as belonging to one of the 45 accepted Clusters of Differentiation (CD). The majority of these clusters define antigens which are restricted to specific leucocyte differentiation lineages or to maturation stages. One group of Mabs, however, belonging to CD 45, define a 200 kilodalton protein, now known as Leucocyte Common Antigen (LCA) or T-200, which has the unusual property of being expressed on virtually on human leucocytes.
There are only a few other molecules with a similar distribution. Class I histocompatibility antigens are found on all leucocytes (with the exception of immature thymocytes), but are also expressed on a wide variety of non-haemopoietic tissue. Two other antigens, identified by Mabs CAMPATH-1 (Blood, 1983; 62:873) and HuLyM3 (Transplantation; 1983 36:446) also have pan-haemopoietic distribution, although their expression on non-haemopoietic cells is less certain.